Imagine if your web browser really knew who you were on the web. Just as you login to your computer, what if when you fired up your browser, it said “Hello Dave” and asked you to “unlock it” as well (Chris Messina was quite influential in my thinking about it this way). In doing so you become securely logged into your OpenID provider (or maybe more than one of them) and as you move around the web your browser takes care of automatically logging you into the sites that you want to be, asking you about others, and helping you register with new ones using your OpenID.
Its not a new concept, as David actually points out.
OpenID for Flock is an add-on that polishes previous attempts of putting OpenID into a browser. While the user experience and graphics are quite a bit better than what I helped build at VeriSign, it's lacking the features that help prevent phishing (making sure you're actually logging into your OpenID provider versus a phishing site that looks like it) which is a bit surprising given Vidoop's involvement. That said, OpenID for Flock is Open Source as part of a project dubbed IDentity in the Browser (IDIB) which the same cannot be said for either Sxipper or VeriSign's OpenID Seatbelt. Given that IDIB is Open Source and already written as a Flock add-on, I'd certainly expect to see it ported to FireFox and there be far more community support of it compared to the other add-ons.
I've been a user of VeriSign's OpenID Seatbeat from day one. Its been super useful but isn't very user friendly. It also does weird things when you open another window. But generally the concept is sound. What I really want is OpenID in the mobile browser, that's even more critical that the desktop browser in my mind.